Re: dlopen deadlock

[Szabolcs Nagy <nsz@port70.net>]

* Rich Felker <dalias@libc.org> [2016-01-14 17:41:15 -0500]:
> On Wed, Jan 13, 2016 at 12:09:37PM +0100, Szabolcs Nagy wrote:
> > This bug i reported against glibc also affects musl:
> > https://sourceware.org/bugzilla/show_bug.cgi?id=19448
> > 
> > in case of musl it's not the global load lock, but the
> > init_fini_lock that causes the problem.
> 
> The deadlock happens when a ctor makes a thread that calls dlopen and
> does not return until the new thread's dlopen returns, right?
> 

yes
(not a common scenario)

> > the multi-threadedness detection is also problematic in
> > do_init_fini:
> > 
> > 	need_locking = has_threads
> > 	if (need_locking)
> > 		lock(init_fini_lock)
> > 	for all deps
> > 		run_ctors(dep)
> > 		if (!need_locking && has_threads)
> > 			need_locking = 1
> > 			lock(init_fini_lock)
> > 	if (need_locking)
> > 		unlock(init_fini_lock)
> > 
> > checking for threads after ctors are run is too late if
> > the ctors may start new threads that can dlopen libs with
> > common deps with the currently loaded lib.
> 
> The logic seems unnecessary now that there's no lazy/optional thread
> pointer initialization (originally it was a problem because
> pthread_mutex_lock with a recursive mutex needed to access TLS for the
> owner tid, but TLS might not have been initialized when the ctors ran)
> but I don't immediately see how it's harmful. The only state the lock
> protects is p->constructed and the fini chain (fini_head,
> p->fini_next) which are all used before the ctors run. The need for
> locking is re-evaluated after the ctors run.
> 

hm ok
i thought the ctors of the same lib might end up being
called twice, concurrently, but i see p->constructed
protects against that

> > one solution i can think of is to have an init_fini_lock
> > for each dso, then the deadlock only happens if a ctor
> > tries to dlopen its own lib (directly or indirectly)
> > which is nonsense (the library depends on itself being
> > loaded)
> 
> The lock has to protect the fini chain linked list (used to control
> order of dtors) so I don't think having it be per-dso is a
> possibility.
> 

i guess using lockfree atomics could solve the deadlock then

> Rich